1. Field of the Invention
The present invention relates to single screw mechanisms such as compressors and expanders specifically and hydraulic motors and pumps generally which are intended to operate on a compressible fluid at high pressure gradients.
2. Description of the Prior Art
The group of single screw compressors which is of concern to the present invention are classified as positive-displacement rotary type machines. This invention relates to single screw mechanisms that utilize a cylindrical mainrotor typically with cylindrical gaterotors. In a machine of this type, there is one mainrotor with a plurality of spiral threads that is driven by prime mover means so as to spin about a fixed axis within a fluid-tight stationary housing. There are usually two gaterotors, which are symmetrically disposed substantially transverse to the axis of the mainrotor, whose teeth are in meshing engagement with the threads of the mainrotor. The housing is provided with inlet and outlet ports for connecting the exterior of this mechanism respectively to a suction and to a discharge plenum. For a conventional machine of the prior art that is intended to raise the pressure of a gas, the process is as follows:
Gas is drawn through the inlet port into the thread of the mainrotor that is open to the suction plenum. After the thread has been filled, a gaterotor tooth rotates into a position where it closes off the filled thread and creates a pocket formed between the mainrotor thread, the gaterotor tooth, and the compressor housing. The mainrotor continues to turn and to decrease the volume of the pocket thereby compressing the entrapped gas. Where cylindrical type gaterotors are employed, depending upon the orientation of such to the mainrotor, either the outside or the inside of the gaterotor tooth can be arranged to sweep out the mainrotor thread. When the desired discharge pressure is reached, the edge of the mainrotor thread uncovers a radial discharge port and the gas is forced into the discharge plenum through this outlet port.
Mechanisms of this type are already well known and have been disclosed, by way of example, in U.S. Pat. Nos. 3,632,239, 4,373,881, and Re. 30,400 among many others. In the first of these, the inventor establishes an algebraic expression for the design of globoid-worm fluid compression-expansion machines which describes mainrotor thread-gaterotor teeth interface relationships and which is satisfied by the several embodiments depicted therein including a conical mainrotor and planar gaterotor geometry. In U.S. Pat. No. Re. 30,400, this same inventor describes a toroidal shaped mainrotor with a transverse mounted gaterotor whose teeth are inclined outward from the axis of said gaterotor and which penetrates the compressor housing through a milled window opening path for engagement. In U.S. Pat. No. 4,373,881, a conical mainrotor having a plurality of helical screw threads is engaged with a cylindrical gaterotor for increased fluid discharge volume by way of increasing the contact length and depth of each gaterotor tooth with each mainrotor groove.
The single screw machines which are available as stock items generally operate at pressures ranging from 60 to 150 psi. This is suitable for most plant systems which are maintained at 90 to 140 psi pressure in order to operate tools and machines which require 70 to 125 psi. However, when higher pressure applications are required, these machines must be multi-staged. This is because the prior art machines' design allow a large number of leakage paths where the trapped fluid can escape from the closed compression chamber. Due to the relative motion between the parts of the machine, the clearance between these parts can only be reduced to minimum finite tolerances. Yet, even when the minimum clearances have been achieved, the volumetric efficiency in the conventional machines has been demonstrated to be closely related to the absolute pressure gradient between the discharge and suction plenums. Therefore, many attempts at reducing the leakage paths have been made and have resulted in various improvements.
In U.S. Pat. No. 3,965,697, labyrinthial seals are incorporated in a lobe type air compressor on the inner wall of the casing bores and on the tips of all of the male lobes to prevent leakage. U.S. Pat. No. 3,945,778 discloses a compression-expansion machine with a cylindrical mainrotor and planar gaterotor geometry which is characterized by gaterotor teeth in which each tooth has two offset rectilinear flanks for obtaining increased compression ratios by reducing leakage past the flank. In U.S. Pat. No. 3,932,077, volumetric discharge is increased by a gaterotor tooth seal which has arc shaped flanks so that surface to surface contact with the mainrotor thread is made within a zone. The major drawbacks of this latter design are that mainrotor thread machining takes a long time and that whenever parallel cylinder profiles are used, leakage past the flank areas becomes major. Recognizing these difficulties, U.S. Pat. No. 4,321,022 proposes gaterotor teeth flanks comprising at least three skewed surfaces which intersect in at least two edges so as to provide dual lines of sealing with the mainrotor thread.
The previously discussed single screw mechanisms are representative of pertinent designs and improvements thereto as disclosed by others when attempting to increase the volumetric efficiency of these machines. Although several embodiments have been discussed, the designs of the prior art contain various limitations and offsetting disadvantages when applied to the high pressure realm so that the effort to reduce leakage remains a major consideration in single screw machinery design. The embodiments hereinafter illustrated and described are distinguishable in the many ways they create better performing machines for operating on a compressible fluid with a high pressure gradient.